Concrete Calculator
Cubic yards and bags for slabs, footings and posts.
Enter your values
Enter the length in ft.
Enter the width in ft.
Enter the thickness in in.
Enter the waste allowance in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Concrete needed
- 2.72 cubic yards
- Cubic feet
- 73.3
- 80 lb bags
- 123
- 60 lb bags
- 163
- Approximate weight
- 11,000 lb
Quick answer
Concrete volume is length × width × thickness converted to cubic yards; one 80 lb bag yields 0.6 cubic feet.
Cubic yards = L × W × T(ft) ÷ 27
At a glance
| What it does | Cubic yards and bags for slabs, footings and posts. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Length, Width, Thickness, Waste allowance |
| Main output | Concrete needed |
| Formula | Cubic yards = L × W × T(ft) ÷ 27 |
| Cost | Free — no sign-up, no download |
How to use the Concrete Calculator
- 1Measure the length and width of the pour.
- 2Enter thickness in inches.
- 3Read cubic yards and bag counts.
Inputs explained
- Length(ft)
- Enter the length in ft.
- Width(ft)
- Enter the width in ft.
- Thickness(in)
- Enter the thickness in in.
- Waste allowance(%)
- Enter the waste allowance in %.
Worked example
Using the values the calculator loads with:
Inputs
- Length20 ft
- Width10 ft
- Thickness4 in
- Waste allowance10 %
Results
- Concrete needed2.72 cubic yards
- Cubic feet73.3
- 80 lb bags123
- 60 lb bags163
- Approximate weight11,000 lb
Frequently asked questions
When should I order ready-mix?
Above about one cubic yard. Mixing 45 bags by hand is slow enough that cold joints form between batches.
How thick should a slab be?
4 inches for patios and walkways, 5–6 inches with rebar for driveways carrying vehicles.
What do I need to enter into the Concrete Calculator?
Just 4 values: length, width, thickness, waste allowance. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Concrete Calculator work out the answer?
Concrete volume is length × width × thickness converted to cubic yards; one 80 lb bag yields 0.6 cubic feet. It applies the formula Cubic yards = L × W × T(ft) ÷ 27 and shows the working so you can check each step by hand.
Is the Concrete Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.